Calix[4]pyrrole dimers bearing rigid spacers: towards the synthesis of cooperative anion binding agents

Calix[4]pyrrole dimers (2, 5, and 6), potential hosts for anionic guests, were synthesized by a procedure involving a palladium(0) catalyzed C–C bond formation process. In the case of 2, a dimer linked via a rigid di-ethynyl spacer, a detailed study of carboxylate anion binding was carried out using 1H NMR spectroscopy. For isophthalate anion, a 1:1 binding stoichometry was observed with this receptor and a much higher association constant was found than for the control monomer, octamethylcalix[4]pyrrole. These findings are ascribed to cooperative binding. Consistent with this conclusion was the finding that, for phthalate and benzoate anions, 1:2 (host:guest) binding stoichometries and lower association constants were recorded than with isophthalate anion. The control compound, octamethylcalix[4]pyrrole, showed a 1:1 binding stoichometry and a much lower association constant than dimer 2, not just with isophthalate but also phthalate and benzoate anions.